The final word on running synthetic in the diffs??

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ryan2022

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Hi guys,

this is going to burn a hole in my head! I've been sitting on the fence about running synthetic in my Jeeps diffs, just to lessen the amount of changes I need to make, and a little more cold weather protection.

I spoke with Currie AND TrueHi9, and they both recommend a 80w140 NON synthetic oil. they were both very decent to take the time to explain to me why they chose a non synthetic oil. Mike from Truehi9 is a excellent guy to chat with if you ever have any questions. But I'm still not convinced its the same for all applications.

Everyone has been jumping on the bandwagon on not using synthetics in the diffs over at Jeepforums.

Where is the truth????? The way I see it, there is a common trend in axle manufacturers that recommend only conventional gear oil. they all make high pinion axles. I could see the synthetic not being drawn up as well or "flung" into the gears as regular oil. BUT in a shallow gear set (like my Dana 44) the gears are bathed in oil, and a dry gear is not really an issue.

What are your thought? Please, try and keep opinons to a minumum and only folks with factual evidence one way or another chime in. Any engineers/ mechanics out there?

Thanks,

Ryan
 
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Synthetic gear oil protects better! My 2007 F-150's rear 9.75" LS Axle came with Synthetic Gear oil from the factory! 75w-140, manual says " Lubed for Life".
 
There is no truth. It is situation dependent.
If it were my diff, I'd use a synthetic and do what I need to make it work.

The 1st issue is that it won't lessen the amount of changes. You have to always maintain your gearboxes and if your vehicle goes swimming, you have to change it out ASAP. Blindly extended maintenance is probably the biggest reason against the word 'synthetic'.

Use what you want and UOA for proof.

There is nothing wrong with a quality mineral gear oil. I myself prefer the 'flow' of a synthetic since we only have 3 warm months and every bit of MPG helps.

Also, if level is low, caused by leaks or whatever, or diff is over worked, I'd rather have the heat resistance of a synthetic to prevent the fluid from oxidizing into sludge.

I also understand that mineral oils sometimes climb a little better and hang around a little longer. The improved 'flow' of a synthetic will cause it to drain back quicker.
In a pumped circuit, there is a benefit as you want as much head on the pump to prevent the oil pump from turning into an air pump.
In a splash/soak situation, maybe a mineral fluid that clings/climbs better and doesn't drain back as quickly is a plus. This might be why many diff's that were filled with 80w90 or SAE90 get 75w140 synth. You need a synth for its benefits but need to go thicker to keep the fluid where you want it.

In a slow speed off road situation, thick and gooey is the way to go and might be why some of the aftermarket 'end manufacturers don't recommend synthetic. Once you're moving on the street/highway, you should not have any problems between mineral or synthetic.

In a slow speed situation, a gear oil pump should be considered. I also wonder if you'd benefit from 'oil snot', moving up to a thicker(190wt/250wt), or overfilling. Time to buy 'clear' plastic or glass(avoid rock crawling) diff covers to compare for yourself.

I don't agree/disagree with the following. But look how well the 80w90 blend climbs compared to the 140wt synthetic in the 2nd picture.
http://www.bobistheoilguy.com/images/lucas/lucas.htm
 
Yeah good call.

I was leaning towards a thicker synthetic as an alternative.

Ive actually got amsoil severe geat 75w110 ready to go in, but I'm a dealer so I dont mind leaving on the shelf for stock.

My Jeep is 95% highway, and daily driven, so it probably falls into that category. Also, I figure with the diffs right out in the open, there is NO shortage of airflow over the pot to help with cooling.

As for the extended drains, I would definetely dump the oil if it was contaminated. I would also most lekely only follow the regular drain schedule in the manual as apposed tot he severe schedule if I ran synthetics.
 
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I'm sure its fine either way - but I have owned many Jeeps over the last 20 years and anytime I change the fluid I install Amsoil gear lube and then I leave it alone for 100k miles. I used to do alot of off-roading and I still do alot of towing. oddly enuf, the only problem I had was not towing my 6000 lb boat behind my 5000lb truck, but rather towing a pontoon boat behind my Jeep Grand Cherokee. it musta been the wind drag or sumthin, but I actually boiled over the gear lube in the rear end. (it was stock dino gear lube). yes, towing a relatively lightweight pontoon boat! the same Jeep used to tow my 3500lb boat all over the country with no problem...
 
Originally Posted By: tomcat27
I'm sure its fine either way - but I have owned many Jeeps over the last 20 years and anytime I change the fluid I install Amsoil gear lube and then I leave it alone for 100k miles. I used to do alot of off-roading and I still do alot of towing. oddly enuf, the only problem I had was not towing my 6000 lb boat behind my 5000lb truck, but rather towing a pontoon boat behind my Jeep Grand Cherokee. it musta been the wind drag or sumthin, but I actually boiled over the gear lube in the rear end. (it was stock dino gear lube). yes, towing a relatively lightweight pontoon boat! the same Jeep used to tow my 3500lb boat all over the country with no problem...


Good to know.

That is proof enough for me!

I think the main issue is the high pinion stuff, and the fact the synthetic doesnt climb as well. On the other hand in a cold environment like mine during the winter, it should still climb better than a frozen solid convention oil!

Theres always a tradeoff, I'm going to toss in the amsoil.

Thanks,

Ryan
 
Originally Posted By: ryan2022
My Jeep is 95% highway, and daily driven...


You kind of answered your own question with this ^.
My YJ is a 95% trail rig that sees beatings. I run 80W90 WalMart gear oil. I have to either change the oil from water crossings or to fix broken parts on the trail. Synthetic would be a costly and unnecessary choice for a trail rig.

Synthetic is the way to go in a daily driver. :cool:
 
My understanding is that they use dino becasue they dip their axles so often in water (and have to change out the fluid) that syn would be a waste. Might not be your situation, but that's what the boys on the Frontier forum do.
 
yeah, that would be one of the main reasons to use it over dino.

Its not usually the case with me though. I like dirt more than water!

Never quite understood the guys that though their Jeep was a submarine.
 
Not the first time I have heard this. 2 guys at Dana who are experts claim dino gear oil "clings & flings" better than syn oil and has better AW properties. They have no data to back it up other than experience with field failures.
21.gif
FWIW I have also read about it from custom diff builders too but I can't remember where.
 
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Thats a good point too. By the looks of it, the gears meshing point is darn near submersed in gear oil. I dont know, Screw it, I'm going to start riding my bike.........now what chain oil to use?
 
There are a number of axle builders who advocate dino over syn and after hearing that, I instigated a discussion over this a couple of years ago and a little private research. We did not come to validate the claims then, and here we are again. The part that bugged me and made me dig is that some of the folks making these claims are skilled and well-known. Too bad they can't come up with a good answer that passes a tribology 101 gut-check.

I started with Dana and Meritor engineers, who had nothing in particular to comment upon. They knew of no engineering reasons why, in a broad sense, syn would be be "bad" when dino would be "good." They did say that often dino was "good enough." One commented, "Lubrication is lubrication! It's either adequate or it's not."

But the axle builders (which included the ones named above and a couple of others) were adamant. The closest thing I got to a cogent answer why was strictly cause and effect. They claimed when synthetic oil, sometimes the gear teeth ended up "polished" and noisy and they were unable to read a pattern on them. They could not explain why this occurred but claimed it did not happen with a dino oil. Made no sense to me and sounded very anecdotal. I queried further to determine if there was a particular brand of oil that did it, but they couldn't (or wouldn't) name any names. One other constant was that this has been their policy for quite some time and I wondered if there wasn't an issue in the old days that is no longer one. You know how past experience can become ingrained.

Finally, I had a long talk with Randy Lyman, of Randy's Ring and Pinion, on this topic. He has been in the axle biz since the '70s, both as a builder and as a parts manufacturer, and has advocated syn oil for many years. We came to the conclusion that, yeah, the polishing thing sometimes happens, but it's often from excessive heat that excessively hardens the surface and that often happens during break-in if the axle is run too hard during that period. It can also happen from excessive hardness in the steel used for the gears (bad heat treat or steel recipe). We speculated that some of these instances may be the result of the improper viscosity being chosen (a 90 when a 140 is needed due to heat).

The most valid reason for dino was mentioned above in other posts. Trail work: Water and having to dump expensive oil too often to be wallet friendly. Plus, I have read that some synthetic base oils do not deal with large amounts of water as well as dinos. I'm not sure I have the exact failure mode nailed down for that circumstance, so I'll remain vague as to details. IIRC, it something about the water causing additive fallout?
 
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Thanks Jim,

thats a good summary.

when I spoke with Currie and Truehi9, they seemed to have the approach to synthetics that many old school mechanics have. a lot of guys in the know are very set in their ways.

It comes back to, if it works dont mess with it, BUT, with the same logic, you might have missed a decade or two in the advancements of a product.

One source mentioned, they had all sorts of trouble from it (synthetic) and also that the seals leaked it everywhere....at that point, I started to wonder how outdated their information was.

I'm not bashing the manufacturers, they were all great guys to take the time to chat, and were all happy to do so.
 
Originally Posted By: ryan2022
Hi guys,

this is going to burn a hole in my head! I've been sitting on the fence about running synthetic in my Jeeps diffs, just to lessen the amount of changes I need to make, and a little more cold weather protection.

I spoke with Currie AND TrueHi9, and they both recommend a 80w140 NON synthetic oil. they were both very decent to take the time to explain to me why they chose a non synthetic oil. Mike from Truehi9 is a excellent guy to chat with if you ever have any questions. But I'm still not convinced its the same for all applications.

Everyone has been jumping on the bandwagon on not using synthetics in the diffs over at Jeepforums.

Where is the truth????? The way I see it, there is a common trend in axle manufacturers that recommend only conventional gear oil. they all make high pinion axles. I could see the synthetic not being drawn up as well or "flung" into the gears as regular oil. BUT in a shallow gear set (like my Dana 44) the gears are bathed in oil, and a dry gear is not really an issue.

What are your thought? Please, try and keep opinons to a minumum and only folks with factual evidence one way or another chime in. Any engineers/ mechanics out there?

Thanks,

Ryan



I'd love to know why anyone recommends AGAINST syn, apart from cost.

To go to the root of the matter, Jeep specifically RECOMMENDS synthetic gear lubricant for the rear axle (Chrysler 8.25 or Dana 35) for towing applications in Cherokees (at least my 1999 Cherokee manual does so).
 
The Jeep Rubicon,both front and rear axles that are Dana 44's,have stamped on the axle itself "75W-140 Synthetic".What does that tell ya!
21.gif
I've always used Pennzoil synthetic on both my Jeeps ,changing them out every couple of years,with no problems.They both have way over 100,000 "real world" miles on them.
 
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I don't have any UOAs or anything, but I do know the full syn Motorcraft 75W140 I put in my old Ranger looked like new after 40K miles. The OEM fluid was dark gray at 55K miles.

On my current truck, I used Valvoline DuraBlend 75W90 when I changed the fluid. The OEM fluid actually looked great and I probably should have just kept using it for a while longer. The DuraBlend doesn't even have 30K on it and it has darkened noticeably. I'm definitely going with a full syn for the next change, which will be soon.

Both trucks have Ford 7.5 axles by the way.
 
I've posted my comment on the anti-synth rumor a few times.

JimAllen brings up a good point.

I've worked in gear manufacturing.
Sometimes a manufacturer ships out a [censored] and its easy to blame a fluid. A bad bearing, bad ring/pinion, bad clearances/tolerance, bad metal, bad bushing, bad assembly...... but the owner used a synthetic so we'll blame that.

If a shop can't give you a direct and honest reason why they have an issue with synthetic, with some kind of proof, then they're are full of ol' timer mentality and are stuck on rumors.
 
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